Objective lens driving device

ABSTRACT

An objective lens driving device of a sliding shaft type is mounted on an optical information recording and reproducing apparatus. The objective lens driving device includes an armature unit having a magnet magnetized in a plurality of pairs of magnetic poles, and a stator unit having a yoke, and control coils mounted on the yoke. The magnet is disposed with the boundaries between the magnetic poles which are respectively opposite gaps formed in the yoke. The objective lens driving device has an improved reliability and a reduced number of component parts.

TECHNICAL FIELD

The present invention relates to an objective lens driving device and,more particularly, to an objective lens driving device employing apermanent magnet and control coils.

BACKGROUND ART

Recently, optical information recording systems capable of stablyrecording a large amount of information at a very high recording densityhave as optical disks, compact disks or external recording equipmentsfor computers.

An objective lens is an essential component of such an optical recordingsystem to focus a light beam in a spot of light on the informationrecording surface of an optical information recording medium to read orwrite information. An objective lens driving device for the positionalcontrol of the objective lens plays a very important role in correctingthe tracking error and the focusing error of the spot of light.

FIG. 1 is an exploded perspective view of a conventional objective lensdriving device.

The objective lens driving device shown in FIG. 1 includes armatureholder 1 and a base yoke 2.

The upper portion of the armature holder 1 is formed in the shape of anoblate hexagon or a rhombus having one end fixedly provided with anobjective lens 3 and the other end fixedly provided with a counterweight4 for counter-balancing the objective lens 3. A focus control coil 5 andtrack control coils 6a, 6b, . . . are disposed in a cylindricalarrangement on the bottom center of the armature holder 1.

The objective lens 3 and the counterweight 4 thus mounted on thearmature holder 1 are fitted respectively in openings 7a and 7b formednear the opposite sides of a substantially square supporting rubbermember 7. The supporting rubber member 7 is fastened to a stator base 9with screws 8a and 8b.

The stator base 9 is provided with a central opening 9a. The controlcoils 5 and 6 disposed on the armature holder 1 project downward throughthe opening 9a. A cover 10 is attached to the upper surface of thestator base 9.

The base yoke 2 includes a holding yoke 13 for holding permanent magnets11a and 11b and a support shaft 12, and yokes 14a and 14b attached tothe upper surfaces of the permanent magnets 11a and 11b, respectively.

The armature holder 1 is mounted on the support shaft 12 so as to beaxially slidable and rotatable relative to the base yoke 2.

FIG. 2 is a longitudinal sectional view of the assembly of the armatureholder 1 and the base yoke 2, in which the parts corresponding to thoseshown in FIG. 1 are denoted by the same reference numerals and thedescription thereof will be omitted.

As is obvious from FIG. 2, a coil holder 15 is disposed on the bottomsurface of the armature holder 1. The focus control coil 5 and the trackcontrol coils 6 are fixed to the coil holder 15.

The armature holder 1 is provided on the bottom thereof with acylindrical bearing 16, which receives the support shaft 12 therein.

The control coils 5 and 6 are fixed to the armature holder 1 as shown inFIG. 3.

The focus control coil 5 is attached to the coil holder provided on thebottom of the armature holder 1, and the track control coils 6a, 6b, 6cand 6d are fixed to the focus control coil 5. Leads 17a, 17b, . . . and17j lead out from the coils.

Motions of the conventional objective lens driving device will bedescribed hereinafter. A light beam, such as a laser beam, is emittedthrough the objective lens 3 so as to be focused in a spot of light onan optical information recording medium, such as an optical disk. Whenthe light beam is out of focus, the objective lens 3 can be focused bysupplying a control current corresponding to a focal displacement to thefocus control coil 5 to slide the armature holder 1 in either of thedirections indicated by a double-head arrow A due to the interactionbetween the control current and the permanent magnets 11. Although thearmature holder 1 is joined through the objective lens 3 and thecounterweight 4 to the supporting rubber member 7, the objective lens 1is slidable because the supporting rubber member 7 is bendable.

When the spot of light deviates from a track, the track control movementof the objective lens 3 can be achieved by supplying a control currentcorresponding to the deviation of the spot of light from the track tothe track control coils 6a, 6b, . . . and 6d to turn the armature holder1 in either of the directions indicated by a double-head arrow B due tothe interaction between the control current and the permanent magnets11.

The conventional objective lens driving device thus constructed has aproblem that it is difficult to manufacture the objective lens drivingdevice.

That is, the leads 17a, 17b, . . . and 17j of the focus control coil 5and the track control coils 6a, 6b, . . . and 6d included in thearmature unit of the objective lens driving device must be connected tothe stator unit, which requires difficult wiring work. Moreover, becausethe armature unit and the stator unit are interconnected by thesupporting rubber member 7 to set the center of a range of track controlmovement in which the armature unit of the objective lens driving devicecan be moved for track control, it is difficult to assemble the armatureunit and the stator unit.

Furthermore, the lead wiring work and the attachment of the supportingrubber must be carried out strictly, or troubles occur in the controlactions of the objective lens driving device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an excellentlyproducible and highly reliable objective lens driving device requiringno work for wiring leads from the armature unit and no work forattaching a supporting rubber member.

To achieve the object, the present invention provides an objective lensdriving device comprising an armature holder for holding an objectivelens, and a driving mechanism for turning and sliding the armatureholder.

The driving mechanism comprises a permanent magnet provided on thearmature holder, a stator yoke disposed opposite to the magnetic polesof the permanent magnet, a focus control coil and track control coilsdisposed in the gap between the permanent magnet and the yoke and fixedto the yoke. The driving mechanism has gaps that are formed in thecircumference of the yoke disposed opposite to the permanent magnet.

The objective lens driving device in accordance with the presentinvention comprises the foregoing components. Accordingly, the permanentmagnet is provided on the armature holder, the control coils arearranged on the yoke, and hence the armature unit need not be providedwith leads. Furthermore, because the gaps are formed in the yoke, themagnetic flux density in the gaps is reduced and a tendency oflow-magnetic-flux-density portions of the annular permanent magnetcoming opposite to the gaps is enhanced. Hence, it is possible to setthe center of a range of track control movement of the armature unit inthe driving device even if the driving device is not provided with anysupporting rubber member.

Thus, the objective lens driving device in accordance with the presentinvention eliminates work for wiring the leads and work for attachingthe supporting rubber member. Thereby, the objective lens driving devicehas excellent producibility and higher reliability.

In the objective lens driving device in accordance with the presentinvention, the permanent magnet is arranged in the stator unitaccordingly, there are no leads extending in the stator unit, and hencethere is no lead extended from the armature unit and no work for wiringleads is necessary.

Furthermore, because the interactions of the gaps formed in the yoke andthe permanent magnet enables the retention of the center of a range oftrack control movement of the armature unit, the armature unit need notbe provided with any supporting rubber member.

Thus, the objective lens driving device eliminates the delicate work forinterconnecting the armature unit and the stator unit, and hence theobjective lens driving device has excellent producibility and highreliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIGS. 1 to 3 are illustrations explaining a conventional objective lensdriving device;

FIGS. 4 to 6 are illustrations explaining the construction of anobjective lens driving device in a first embodiment according to thepresent invention;

FIGS. 7 and 8 are illustrations explaining the retaining function of thecenter of track control movement of the objective lens driving device inthe first embodiment;

FIGS. 9 to 11 are illustrations explaining the retaining function of thecenter of focus control movement of the objective lens driving device inthe first embodiment;

FIGS. 12 to 16 are illustrations explaining a modification of theretaining function of the center of focus control movement of objectivelens driving device in the first embodiment;

FIGS. 17 to 19 are illustrations explaining the construction of anotherobjective lens driving device in a second embodiment according to thepresent invention; and

FIG. 20 is an illustration explaining the retaining function of thecenter of track control movement of the objective lens driving device inthe second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 4 is an exploded perspective view of an objective lens drivingdevice in a first embodiment according to the present invention, inwhich parts corresponding to those of the foregoing conventionalobjective lens driving device are denoted by reference numerals obtainedby adding 100 to those of corresponding parts and the descriptionthereof will be omitted to avoid duplication.

The present invention has a permanent magnet that is arranged on anarmature holder, control coils are arranged on a yoke of a stator unit,and gaps that are formed in a base yoke at positions facing thepermanent magnet.

Accordingly, an armature holder 101 employed in an objective lensdriving device in the first embodiment is provided with a cylindricalbearing 116 near the central portion thereof. An objective lens 103 isfixed to the armature holder 101 at an eccentric position at apredetermined distance from the bearing 116. An annular skirt 130extends downward from the bottom of the movable holder 101, and anannular permanent magnet 111 magnetized radially and symmetrically infour magnetic poles is fixedly held on the annular skirt 130. Thepolarity of the poles 111a and 111b of the annular permanent magnet 111on the same circumference is the same, and the polarity of the poles111c and 111d of the annular permanent magnet 111 on the samecircumference is the same.

On the other hand, the circumferential wall of a substantiallycylindrical base yoke 102 is divided into two portions forming outerprojections 132a and 132b. A support shaft 112 extends upright from thesubstantially central portion of the base yoke 102. An inner projection134 is formed concentrically with the outer projections 132a and 132bbetween the outer projections 132a and 132b, and the support shaft 112.

Gaps 136a and 136b are formed symmetrically between the outerprojections 132a and 132b so as to coincide respectively with theboundary between the poles 111a and 111c of the annular permanent magnet111 and the boundary between the poles 116b and 116d of the annularpermanent magnet 111.

A focus control coil 105 is disposed around and fixed to the outercircumference of the inner projection 134. Track control coils 106a and106b respectively wound on back yokes 138a and 138b are fixedly disposedrespectively in the gaps 136a and 136b. The track control coils 106a and106b have each a rectangular shape and are disposed so that thedirections of vectors of magnetic flux intersecting with each other onone of the sides substantially parallel with the axis of the supportshaft 112 and the other side are substantially opposite to each other.

In assembling the objective lens driving device having the aforesaidconstruction, the support shaft 112 fixed to the base yoke 102 isinserted in the bearing 116 of the armature holder 101 as shown in FIGS.5 and 6.

FIG. 5 is a top plan view of the objective lens driving device in thefirst embodiment according to the present invention, and FIG. 6 is alongitudinal sectional view taken on line VI--VI in FIG. 5.

The operation of the objective lens driving device in the firstembodiment thus constructed will be described hereinafter.

Similar to the operation of the foregoing conventional objective lensdriving device, a laser beam is emitted through the objective lens 103and focused in a spot of light on the information recording surface ofan optical information recording medium, such as an optical disk. Whenthe spot of light is out of focus, a control current corresponding to afocal displacement is supplied to the focus control coil 105 to slidethe armature holder 101 through the permanent magnet 111 in either ofthe directions indicated by a double-head arrow A for the focus controlmovement of the objective lens 103.

When the spot of light deviates from a track, a control currentcorresponding to the deviation of the spot of light from the track issupplied to the track control coils 106a and 106b to turn the armatureholder 101 through the permanent magnet 111 in either of the directionsindicated by a double-head arrow B for the track control movement of theobjective lens 103.

Because the focus control coil 105 and the track control coils 106a and106b are provided on the base yoke 102, namely, a stator unit, andcurrent need not be supplied to the armature holder 101, namely, anarmature unit, wiring work for the leads can be achieved very easily.

Moreover, because the annular permanent magnet 111 is disposed so thatthe boundaries between the poles 111a and 111c and between the poles111b and 111d of the annular permanent magnet 111, coincide respectivelywith the gaps 136a and 136b formed in the base yoke 102, the magneticflux gradient on the circumference of the permanent magnet 111 changeswhen the movable holder 101 is turned in either of the directions of thedouble-head arrow B and a restorative force tending to restore thearmature holder 101 to the original position acts on the permanentmagnet 111. As a result, the center of track control movement of thearmature holder 101 can be retained.

Accordingly, any supporting rubber member, which is necessary for theconventional objective lens driving device, is not necessary, and hencethe assembling work is facilitated and no problem occurs in the controloperation of the objective lens driving device.

Incidentally, when the permanent magnet 111 is disposed so that theboundaries between the pole faces thereof coincide respectively with thegaps 136a and 136b in FIG. 7, which show the disposition of the baseyoke and the permanent magnet, it is preferable for the stable retentionof the center of track control movement that the central angle θ_(m) ofeach of the poles 111c and 111d of the annular permanent magnet 111 andthe central angle θ_(y) of each of the gaps 136a and 136b of the baseyoke 102 meet an inequality: θ_(m)≧θy.

It is also possible to retain the center of track control movement whenthe poles 111c and 111d of the annular permanent magnets 111 aredisposed opposite to the gaps 136a and 136b, respectively, as shown inFIG. 8. In this case, the central angles θ_(m) and θ_(y) must meet aninequality: θ_(m<)θy.

An improvement of the performance in retaining the center of trackcontrol movement of the armature holder will be described hereinafterwith reference to FIGS. 9 through 16.

FIGS. 9 to 11 are illustrations explaining the objective lens drivingdevice in the first embodiment, and FIGS. 12 to 16 are illustrationsexplaining an objective lens driving device in a modification, in whichparts corresponding to those shown in FIG. 4 are denoted by the samereference numerals and the description thereof will be omitted.

First, the modification will be described with reference to FIGS. 12through 16. As shown in FIG. 12, coil seats 150 are disposed fixedly onthe inner circumference of a base yoke 102 between outer projections132a and 132b, and an inner projection 134. Track control coils 106a,106b, 106c and 106d are positioned respectively in recesses 151a, 151b,151c and 151d formed in the inner surfaces of the coil seats 150.

The track control coils 106a, 106b, 106c and 106d have each arectangular shape. The track control coils 106a and 106b are disposedadjacently, while the track control coils 106c and 106d are disposedadjacently. A permanent magnet 111 is disposed so that a predeterminedgap is formed between the permanent magnet 111 and a focus control coil105 and a predetermined gap is formed between the permanent magnet 111and the track control coils 106a, 106b, 106c and 106d. The permanentmagnet 111 is magnetized so that the magnetic polarity of a surfacefacing a portion adjacent to the sides of the track control coils 106a,106b, 106c and 106d is different from that of another surface on thesame circumference.

Because this objective lens driving device is thus constructed, thedistribution of the magnetic flux density (magnetic flux gradient) onthe surface of the permanent magnet 111 changes and a restorative forceF acts on the permanent magnet 111 when the permanent magnet 111 ismoved in the directions of the arrow A (FIG. 12) as shown in FIGS.15(A), 15 (B) and 15(C) for focus control.

The restorative force F acts downward (leftward in FIG. 15) in theobjective lens driving device. The restorative force F varies graduallyas the permanent magnet 111 moves toward the upper open end of the baseyoke 102, and increases sharply after the permanent magnet 111 has movedoutward beyond the upper end of the base yoke 102, namely, a positionshown in FIG. 15(B).

As is obvious from FIG. 16, even when the permanent magnet 111 is at aposition shown in FIG. 15(A), namely, even within the base yoke, alittle restorative force F acts on the permanent magnet 111.

This restorative force F acting on the permanent magnet 111 makes themovement of the objective lens driving device nonlinear as shown in FIG.16, which may cause inconvenience to control the movement of theobjective lens driving device.

In the first embodiment, i.e., a basic embodiment, back yokes 138a and138b are disposed opposite to the poles of the permanent magnet 111 asshown in FIG. 9 in an enlarged view. Accordingly, when the permanentmagnet 111 is moved in the direction of an arrow X1 for focus control,the restorative force F tending to pull back the permanent magnet 111acts on the permanent magnet 111 in the direction of an arrow X2 asshown in FIG. 10.

On the other hand, as shown in FIG. 10(C), when the permanent magnet 111is moved in the direction of the arrow X2, the restorative force F actson the permanent magnet 111 in the direction of the arrow X1. Therestorative force F, as obviously shown in FIG. 11, acts in the oppositedirection to that of movement of the permanent magnet 111.

Thus, in the configuration shown in FIG. 9, the back yokes 138a and 138bhaving a height H_(y), and the permanent magnet 111 having a heightH_(m) are disposed opposite to each other to retain the permanent magnet111 at the center corresponding to the center of the height of the backyokes 138a and 138b.

Such central position retention is possible because a magnetic gradientproduced by the permanent magnet 111 and the back yokes 138a and 138bwhen the permanent magnet 111 is moved in the direction of the arrow X1by a fixed distance δx, and a magnetic gradient produced by thepermanent magnet 111 and the back yokes 138a and 138b when the permanentmagnet 111 is moved in the direction of the arrow X2 by the fixeddistance δx are symmetrical.

In retaining the center of focus control movement with the permanentmagnet 111 and the back yokes 138a and 138b, it is appropriate todetermine the values of the heights H_(m) and H_(y) so that aninequality H_(m)≦ H_(y) is satisfied.

An objective lens driving device in a second embodiment according to thepresent invention will be described hereinafter with reference to FIGS.17 through 20, in which parts corresponding to those of the firstembodiment will be denoted by the same reference numerals and thedescription thereof will be omitted to avoid duplication.

The device in the second embodiment has recesses are formed in thepermanent magnet 111. Referring to FIG. 17, the annular permanent magnet111 has S-poles on the outer circumference thereof, N-poles on the innercircumference thereof, and recesses 140a and 140b formed diametricallyin the outer circumference thereof. The permanent magnet 111 is disposedwith the recesses 140a and 140b thereof positioned respectively oppositethe gaps 136a and 136b of the outer projection 132.

In the second embodiment, the track control coils 106 are woundrespectively on the back yokes 138 and are disposed fixedly andrespectively in the gaps 136a and 136b of the outer projection 132 ofthe base yoke 102 opposite to the permanent magnet 111.

The operation of the objective lens driving device in the secondembodiment thus constructed will be described hereinafter.

Similar to the operation of the first embodiment, a desired controlcurrent is supplied to the focus control coil 105 to slide the armatureholder 1 in either of the directions of a double-head arrow A (FIG. 19)for the focus control movement of the objective lens 3.

A desired current is supplied to the track control coils 106a and 106bto turn the armature holder 1 in either of the directions of adouble-head arrow B (FIG. 18) for the track control movement of theobjective lens 103.

In the second embodiment, because the permanent magnet 111 is disposedwith the recesses 140a and 140b thereof respectively opposite the gaps136a and 136b of the base yoke 102, the magnetic flux gradient on thecircumference of the permanent magnet 111 changes when the armature unitrepresented by the armature holder 101 is turned in either of thedirections of the arrow B, and then a restorative force tending torestore the armature unit to its original position acts on the permanentmagnet 111, which enables the retention of the center of track controlmovement of the armature holder 101. In retaining the center of trackcontrol movement of the armature holder 101 with the recesses 140a and140b of the permanent magnet 111, and the gaps 136a and 136b in the baseyoke 102, it is preferable for the stabilization of the drivingefficiency and the retention of the center to determine the values ofthe central angle θ_(m) of the recesses 140a and 140b of the permanentmagnet 111 and the central angle θ_(y) of the gaps 136a and 136b shownin FIG. 20 so that an inequality θ_(y)≧θm is satisfied.

As is apparent from the foregoing description, the objective lensdriving device in accordance with the present invention is useful as alens positioning mechanism for controlling the objective lens of anoptical information recording system to correct the focusing andtracking errors of the objective lens. Particularly, the objective lensdriving device eliminates unnecessary members to facilitate theassembling work, is improved in reliability, and is suitable for thehighly accurate positioning of the lens.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. An objective lens driving device comprising:an armatureholder rotatably and slidably supported by a support shaft; an objectivelens mounted on said armature holder at an eccentric position withrespect to the support shaft; and a driving mechanism for driving saidholder in turning and sliding motions to control focusing and trackingof a spot of light focused on an optical information recording mediumthrough the objective lens, said driving mechanism including, apermanent magnet provided on said holder, a base yoke of a stator unitdisposed opposite to the pole surfaces of said permanent magnet andhaving gaps formed in the circumference thereof so as to extend inparallel with the support shaft, track control coils for tracking thespot of light on an information recording track formed in opticalinformation recording medium by turning said holder, disposed in a spaceformed including the gaps including the gaps of said base yoke, andfixed to said base yoke, and a focus control coil for sliding saidholder along the support shaft to focus the light beam in a spot oflight on the information recording surface of the medium, disposed inthe space, and fixed to said base yoke.
 2. An objective lens drivingdevice according to claim 1, wherein said permanent magnet is an annularpermanent magnet magnetized in directions substantially perpendicular tothe axis of the support shaft with a plurality of pairs of magneticpoles, the gaps formed in the circumference of said base yoke confrontat least one boundary between the magnetic poles of said permanentmagnet.
 3. An objective lens driving device according to claim 2,wherein the boundaries between the magnetic poles of said annularpermanent magnet and the gaps formed in said base yoke are arrangedsymmetrically with respect to the axis of the support shaft, and saidannular permanent magnet has at least four boundaries.
 4. An objectivelens driving device according to claim 2, wherein said annular permanentmagnet is disposed with one of the boundaries between the magnetic polesopposite to the gaps formed in said base yoke.
 5. An objective lensdriving device according to claim 2, wherein said annular permanentmagnet is disposed with two of the boundaries between the magnetic polesthereof opposite to the gaps formed in said base yoke.
 6. An objectivelens driving device as in claims 1-5 wherein said focus control coil andsaid track control coils are disposed within the space between saidpermanent magnet and said base yoke so that effective magnetic fluxesintersect with each other.
 7. An objective lens driving device accordingto claim 2, wherein said permanent magnet has an annular shape, the yokesurface of said base yoke facing said permanent magnet is concentricwith said permanent magnet, the gaps are provided in the direction ofthe outer circumference of said permanent magnet, and plate-shaped yokesare fixedly arranged on the outer circumference of said base yoke at thegaps opposite said permanent magnet.
 8. An objective lens driving deviceaccording to claim 7, wherein said focus control coil is formed in theshape of a ring and is disposed fixedly so that a current flowstherethrough in the direction of the inner circumference of saidpermanent magnet and perpendicularly to the axis of the support shaft,and said track control coils are formed in a rectangular shape and aremounted on the plate-shaped yokes opposite to said permanent magnet sothat some sides thereof are substantially parallel with the axis of thesupport shaft.
 9. An objective lens driving device according to claims 7or 8, wherein the gaps are fixedly and symmetrically arranged on saidbase yoke, and the plate-shaped yokes are fixedly and symmetricallyarranged on said base yoke.
 10. An objective lens driving deviceaccording to claim 7, wherein said focus control coil is formed in theshape of a ring and is disposed fixedly so that a current flowstherethrough in the direction of the inner circumference of saidpermanent magnet and perpendicularly to the axis of the support shaft,said track control coils are formed in a rectangular shape, and theplate-shaped yokes are fixed to said base yoke opposite to saidpermanent magnet with some of the sides thereof substantially inparallel with the axis of the support shaft.
 11. An objective lensdriving device according to claim 10, wherein the gaps are formedsymmetrically in said base yoke, and the plate-shaped yoke is fixedlyand symmetrically arranged on said base yoke opposite to said permanentmagnet.
 12. An objective lens driving device according to claim 1,wherein said permanent magnet is formed in the shape of a ring, providedin the circumference thereof with recesses, and is disposed with therecesses opposite the gaps formed in the yoke surface of said base yoke.13. An objective lens driving device according to claim 12, wherein therecesses formed in said annular permanent magnet are arrangedsymmetrically with respect to the axis of the support shaft, and thegaps formed in said base yoke are symmetrical with respect to the axisof the support shaft.
 14. An objective lens driving device according toclaims 12 or 13, wherein said annular permanent magnet is magnetizedradially substantially perpendicular to the axis of the support shaft.15. An objective lens driving device according to claims 12 or 13,wherein said track control coils are wound on said plate-shaped yokes,and the plate-shaped yokes are provided fixedly in the gaps in said baseyoke so that a current flows through said track control coils in thedirection of the axis of the support shaft.
 16. An objective lensdriving device according to claims 12 or 13, wherein said annularpermanent magnet is magnetized radially substantially perpendicular tothe axis of the support shaft, said track control coils are wound on theplate-shaped yokes, and the plate-shaped yokes are provided fixedly inthe gaps in said base yoke so that a current flows through said trackcontrol coils in the direction of the axis of the support shaft.
 17. Anobjective lens driving device according to claim 3, wherein said annularpermanent magnet is disposed with two of the boundaries between themagnetic poles thereof opposite to one of the gaps formed in said baseyoke.